A rotary hearth furnace includes an outer circumference wall, an inner circumference wall, and a rotary hearth which is arranged between the walls. The rotary hearth includes an annular hearth frame, a hearth heat insulating material which is arranged on the hearth frame, and a refractory which is arranged on the hearth heat insulating material.
Such a rotary hearth is rotated by a driving mechanism. With respect to the driving mechanism, for example, there are a gear mechanism in which a pinion gear driven by a rotary shaft provided to a lower part of the furnace engages with a rack rail which is circumferentially fixed to a bottom part of the hearth frame, and a mechanism in which a plurality of drive wheels provided to the bottom part of the hearth frame drive on a track which is circumferentially provided on a floor.
The rotary hearth furnace which has such a structure is used for metal heating process of a steel billet and the like or combustion process of flammable waste, for example. In recent years, methods of producing reduced iron from iron oxide by using the rotary hearth furnace have attracted notice.
Hereinafter, with reference to a schematic view illustrating a known rotary hearth furnace illustrated in FIG. 5, an example of reduced iron production process by the rotary hearth furnace will be described.
(1) Powdered iron oxide (iron ore, electric furnace dust, etc.) and powdered carbonaceous reducing agents (coal, cokes, etc.) are mixed and palletized to form green pellets.
(2) The green pellets are heated up to such a temperature area that combustible volatile components generated from the pellets may not ignite to remove contained moisture to obtain dry pellets (raw material 29).
(3) The dry pellets (raw material 29) are supplied into a rotary hearth furnace 26 using a suitable charging unit 23. Then, a pellet layer which has a thickness of about one to two pellets is formed on a rotary hearth 21.
(4) The pellet layer is radiant heated for reduction by combustion of a burner 27 installed to an upper part of the inside of the furnace to metalize.
(5) The metalized pellets are cooled by a cooler 28. The cooling is performed, for example, by directly spraying gas on the pellets or indirectly cooling by a cooling water jacket. By cooling the pellets, mechanical strength endurable for handling at a time of discharge and after the discharge is obtained. Then, the cooled pellets are discharged by a discharge unit 22.
(6) After the metalized pellets (reduced iron 30) are discharged, the dry pellets (raw material 29) are immediately charged and by repeating the above process, reduced iron is produced.
The rotary hearth furnace has a lower part heat insulation structure that is composed of an annular hearth frame, a heat insulation material layer which is arranged on the hearth frame, and a refractory layer which is arranged on the heat insulation material layer. To an outer circumference side and an inner circumference side of the rotary hearth, an outer circumference side corner refractory and an inner circumference side corner refractory are arranged through hearth curb castings respectively.
At a time of operation of the rotary hearth furnace, to an upper part of the lower part heat insulation structure which is surrounded by the outer circumference side and the inner circumference side corner refractories of the rotary hearth, surface materials such as a mixture of dolomite, iron ore, iron oxide (iron ore, electric furnace dust, etc.), carbonaceous reducing agents (coal, cokes, etc), or a material to be processed are charged and reduction process is performed.
Accordingly, due to the difference among these materials which constitute the rotary hearth, interference among the lower part heat insulation structure, the corner refractories, and the surface materials becomes complicated, and in some cases, the corner refractories or the lower part heat insulation structure may be damaged.
Especially, although there is no problem on the surface material during construction of the rotary hearth furnace before the rotary hearth furnace is operated, once the rotary hearth furnace is operated and continuously used for a long period, the dolomite and the iron ore accumulates, solidifies, and becomes unified. The unified dolomite and iron ore often circularly solidifies at a furnace outer circumference part and sometimes the solidified material is formed all over the furnace. If the rotary hearth furnace is cooled after the furnace surface is unified as described above, the refractories and the heat insulating materials are contracted and this causes gaps or cracks.
To the layer of the dolomite and the iron ore which is to be a surface layer, it is not possible to intentionally provide an expansion margin, and thus, cracks at points where the cracks most likely to occur and contracts by itself. If the surface layer is heated up again, the surface layer does not always return to the state before the cooling, there are many parts affected by external force due to thermal expansion. The external force due to the thermal expansion acts not only in a circumferential direction, but acts in a radius direction.
On the other hand, the hearth frame is structured to contract, however, when heated again, as a matter of course, because the hearth frame is heated up from an upper part, during nonsteady temperature increase to a steady state in the furnace temperature, a phenomenon that only members in the upper part expand occurs. By the phenomenon, the corner refractory provided at an end part of the inner circumference side or the outer circumference side of the rotary hearth is pushed, and may fall to the outside of the furnace, may be floated, or a fixing metallic material may be damaged. Known examples in which the above-described problems have been improved are described with reference to FIGS. 6 and 7.
FIG. 6 is a fragmentary plane view illustrating a hearth structure of a known rotary hearth furnace. In the hearth structure, an annular rotary hearth 52 is arranged between an inner circumference wall and an outer circumference wall, and an intermediate part of the rotary hearth 52 in an inner-outer direction is constituted of a refractory castable layer 55. On at least one of the inner circumference side or the outer circumference side of the refractory castable layer 55, a plurality of rows of refractory bricks 73 and 74 are adjacently arranged in the inner-outer direction to form predetermined gaps 57 and 58 between the rows of refractory bricks 73 and 74.
Moreover, a rotary hearth furnace according to another known example is described with reference to fragmentary schematic view 7 illustrating a cross section of the rotary hearth furnace. The rotary hearth furnace includes a hearth central body 35 which has a rotatable hearth frame 32, a heat insulating brick 33 which is arranged on the hearth frame 32, and a castable refractory 34 which is arranged on the heat insulating brick 33. The rotary hearth furnace is constituted of refractories, and includes a hearth inner-outer circumference position determination part 37 which is arranged on the hearth frame 32.
In the rotary hearth furnace, to an inner-outer circumference part of the heat insulating brick 33 of the hearth central body 35, a step part 38 is formed using the same heat insulating brick and an expansion margin 39 is provided between the heat insulating brick which forms the step part 38 and the castable refractory 34 which is arranged inside of the step part 38. The expansion margin 39 is provided in a size of 25 mm or more, preferably, 30 mm.
To the hearth inner-outer circumference position determination part 37, a castable refractory 40 is provided. To an outer circumference of the castable refractory 40, an L-shaped metallic material 41 which is fixed to the hearth frame 32 is arranged. On the castable refractory 40, a position determination refractory 42 which is formed by layering an inorganic fiber heat insulating material is provided. The position determination refractory 42 is fixed to the castable refractory 40.
However, in the conventional rotary hearth furnace described with reference to FIG. 6, there is no specific description how much the size of the gaps 57 and 58 formed as the thermal expansion margins should be.
On the other hand, in the known example described with reference to FIG. 7, the specific size of the expansion margin 39 is described. However, the size of the expansion margin 39 is the size compensated according to the calculation if the width of the castable refractory 34 is 2825 mm, it is not possible to apply the known example to a case in which a size of a furnace or a material constituting the furnace is different. Accordingly, the known example cannot be a guiding technique which shows how to determine the expansion margin. Further, in any of the above-described known examples, there is a problem that the furnace structures are too complicated and therefore, the construction is difficult and the costs increase.
In the rotary hearth furnace, at a time of heating, the temperature increases to 500° C. or more, and in some cases, increases to 600° C. or more. Then, by external force due to thermal expansion which acts on the corner refractories, force in a lateral direction acts on the corner refractory hearth curb castings which supports the corner refractories. Accordingly, it is necessary to use expensive alloy, for example, alloy corresponding to ASTM HH, for the corner refractory hearth curb castings. However, there is a problem that the alloy is short in the life.